Below is a list of known to be active Haskell web frameworks. Rather than one framework to rule them all, Haskell provides several options. You can view the Web/Deploy page to get an idea of how you might deploy an application written in some of these frameworks.

Happstack is primarily intended for use on VPS or dedicated hosts, but can be used with CGI via FastCGI or hack.

See the Happstack Home Page for more information and to learn how to get support via IRC and mailing lists.

2 Snap

Snap is a simple web development framework for unix systems, written in the Haskell programming language.

Snap is well-documented and has a test suite with a high level of code coverage, but it is early-stage software with still-evolving interfaces. Snap is therefore likely to be most appropriate for early adopters and potential contributors.

A fast HTTP server library with an optional high-concurrency backend using the libev event loop library

3 Yesod

Yesod is designed for RESTful, type-safe, performant web apps. By leveraging quasi-quotation for the more boilerplate tasks, we get concise web apps with high levels of type safety. Its Hamlet templates are compile-time checked for correctness, and the controller (web-routes-quasi) uses type-safe URLs to make certain you are only generating valid URLs. It loosely follows Model/View/Controller principles.

Yesod is a full-featured web framework. It takes a modular approach to development, so many parts of the framework such as Hamlet and Persistent are available as standalone packages. However, put together, Yesod provides you with solutions for templating, routing, persistence, sessions, JSON, authentication/authorization, and more. Yesod's major guiding principle is type safety: if your application compiles, it works.

Yesod is very well documented through the Yesod book. Work is being done on an constant basis to improve the documentation status, but the first ten chapters (covering all the basics) are already done, so it should be easy to get started.

Yesod is built on WAI, or the Web Application Interface. This is similar to WSGI in Python or Rack in Ruby. It provides a single interface that all applications can target and work on multiple backends. Backends exist for CGI, FastCGI, SCGI, development server (auto-recompile) and even a Webkit-powered desktop version.

But the premier backend is Warp: a very simple web server which, at the time of writing, is the fastest Haskell has to offer. You can read more in its release announcement and see some followup benchmarks. Warp is already powering Yesod; some other major players that are planning a move are Hoogle and Happstack.

4 Haskell on a Horse

Haskell on a Horse (HoH) is a combinatorial web framework for the programming language Haskell. It is currently at an early, unsettled stage of development. It is available under the "BSD3" open-source license.

6 Lemmachine

Lemmachine is a REST'ful web framework that makes it easy to get HTTP right by exposing users to overridable hooks with sane defaults. The main architecture is a copy of Erlang-based Webmachine, which is currently the best documentation reference (for hooks & general design).

Lemmachine stands out from the dynamically typed Webmachine by being written in dependently typed Agda. The goal of the project is to show the advantages gained from compositional testing by taking advantage of proofs being inherently compositional. See proofs for examples of universally quantified proofs (tests over all possible input values) written against the default resource, which does not override any hooks.

8 Salvia

Salvia is a feature rich modular web server and web application framework that can be used to write dynamic websites in Haskell. From the lower level protocol code up to the high level application code, everything is written as a Salvia handler. This approach makes the server extremely extensible. To see a demo of a Salvia website, please see the salvia-demo package.

All the low level protocol code can be found in the salvia-protocol package, which exposes the datatypes, parsers and pretty-printers for the URI, HTTP, Cookie and MIME protocols.

This Salvia package itself can be separated into three different parts: the interface, the handlers and the implementation. The interface module defines a number of type classes that the user can build the web application against. Reading the request object, writing to the response, or gaining direct access to the socket, all of these actions are reflected using one type class aspect in the interface. The handlers are self contained modules that implement a single aspect of the Salvia web server. The handlers expose their interface requirements in their type context. Salvia can have multiple implementations which can be switched by using different instances for the interface type classes. This package has only one implementation, a simple accepting socket loop server. The salvia-extras package has two additional implementations. Keeping a clear distinction between the abstract server aspects and the actual implementation makes it very easy to migrate existing web application to different back-ends.